1. Field of the Invention
The present invention relates to a synchronizing system for use in a digital communication line and, in particular, to an improved synchronizing system which controls the synchronism of the whole system that is composed of a plurality of button telephone main exchanges each having a digital communication line or a plurality of private branch exchanges (PBXs) connected together by means of links
2. Description of the Prior Art
A digital communication network, normally, includes a main exchange and a large number of local or branch exchanges. The main exchange includes a reference clock generator and each branch exchange receives a reference clock from the main exchange and is synchronized with the reference clock before it is operated. Such network in which the branch exchanges are dependent on the clock source of the main exchange is composed of a plurality of stages. In such a staged structure network, there is employed a continuously synchronous system in which a reference clock is always supplied to an exchange belonging to a certain stage from another stage which is higher by 1 than the former stage.
On the other hand, the digital communication network may sometimes request a call-by-call synchronous system of a communication device such as a PBX (private branch exchange) and the like connected to such digital communication network, in which call-by-call synchronous system a reference clock is supplied each time a call is generated from the digital communication network as in a basic circuit employed in an ISDN (Integrated Service Digital Network).
When a main exchange and a large number of local or branch exchanges respectively connected to the digital communication network of the above-mentioned call-by-call type are connected to each other in a star connection manner, conventionally, there are available two kinds of systems: that is, in one of the two systems the main exchange is in synchronism with the digital communication network and the branch exchanges are synchronized dependent on the main exchange; and, in the other system, the branch exchanges are in synchronism with the digital communication network and the main exchange is synchronized dependent on the branch exchanges.
In the conventional system that includes a main exchange and a large number of branch exchanges each of which main and branch exchanges can be connected to a digital communication network, however, a clock source which serves as a reference each time a call is made cannot be moved quickly, and a synchronizing frequency in a mutual synchronized state occurring while the clock source is being moved must be kept within a desired accuracy without being influenced by a delayed time in a link transmission line; that is, in the conventional system there are left the above problems to be solved.